1. Field of the Invention
The present invention relates to hydraulic couplings. More specifically, the field of the invention is that of hydraulic port fittings such as used in automobiles, aircraft, and the like.
2. Related Art
There are many port fittings on the marketplace currently which are designed to connect tubing or hydraulic hose to power equipment such as brake calipers etc. The most frequently used methods of attachment are the tube-to-port and the "banjo" type fittings. The former is used in brake, power steering, and air conditioning systems, and the latter predominantly in brake systems.
The problem with both technologies is that they are prone to leaks which are both expensive and dangerous. Warranty and in-house costs of fixing such leaks may become quite large, and the environmental consideration of the effects of such loss of contaminating fluids cannot be ignored.
Traditionally, the hydraulics supply industry has recommended better surface finishes and tighter tolerances in manufacture in order to minimize the potential for leaks. This has, however, failed to adequately answer the problem. To adequately address this problem, the causes of the leaks in two prior art port fittings must be examined. Descriptions of the tube-to-port and "banjo" port fittings are presented below which include an examination of their inherent inadequacies.
TUBE-TO-PORT FITTINGS
The prior art tube-to-port fitting, as shown in FIG. 1, comprises a tube 1 which includes a raised bead 2 which sits behind reduced diameter portion 3. This diameter 3 is required to be of very smooth surface finish. Often, the tube-to-port fitting also includes an o-ring retention feature 4. The periphery of bead 2 is severely stressed during the formation process, and these stresses can lead to cracking, though proper specification of tubing composition may ameliorate this problem. However, specification of required chemical composition, heat treatment, hardness, wall thickness, or specific manufacturing methods for the tubing further adds to the expense of the fitting.
Tube 1 is assembled to a mating port 5 with tube nut 6. The port 5 has a very finely machined internal configuration which is required to have a fine surface finish in order to seal reliably.
When assembled with the required o-ring 11, as in FIG. 2, the bead on the tube portion comes into tight axial abutment with the flat-faced feature 9 in the bottom of port 5. O-ring 11 is driven down taper 10 of port 5 and forms a seal in the tapered area and along a portion of the parallel interface between surfaces 7 and 8.
Although not immediately apparent why a coupling of this design should be the source of so many leaks, detailed analysis reveals that the interface is not ideal for reliable long-term service. The following paragraphs describe problems inherent in the prior art tube-to-port fitting.
One problem involves the abutment of tube bead with flat-bottomed base of the port which is essentially face-to-face, wherein the bead is put into a plastic deformation by the pressure exerted by the tube nut. There is very little elastic resiliency in the interface. Once the metal of the tube takes a set after assembly, subsequent pressurization forces, vibration, flexure, heating and cooling, etc. may give rise to a gap which will only worsen over time.
Another problem involves the orientation of the o-ring. Positioned in both the tapered area and the parallel interface, the o-ring is deformed during the assembly operation into a kidney-like shape, with a portion remaining in the relatively large tapered portion of the port. When the o-ring takes a set over time, it will be incapable of movement within the pocket, being trapped by the tapered portion. In order for an o-ring to work properly, it must be capable of moving in response to pressure differentials. Therefore, the improper positioning of the o-ring in the port interface tends to prevent the o-ring from moving and thus degrades long term performance of the fitting.
An additional problem involves the fact that the only seal in the tube-to-port fitting is provided by the o-ring. Without the o-ring in place, the tube does not seal when assembled to the port, even against low pressures. Also, misalignment of components may result in a damaged o-ring, and with this frequent problem a leak will be immediately apparent.
A further problem exists in a situation where the tube-to-port fitting is used in air conditioning systems, due to the searching nature of refrigerants. O-rings are permeable to freon, especially under pressure. If the o-ring is the only seal in a system, a constant and irretrievable loss of refrigerant to the atmosphere occurs through the permeable o-ring. This loss is slow at first, due to the slowness of permeation through the o-ring, but it becomes more rapid as the o-ring takes a set over time, and larger leak-paths occur.
"BANJO" FITTINGS
"Banjo" fittings are so termed because of their shape, which usually comprises of a tube brazed onto a round component, giving rise to a substantially banjo-shaped assembly.
Referring to FIGS. 3 and 4, which show a prior art "banjo" fitting, a typical banjo fitting is comprised of body 102 through which bolt 103 is assembled. The combination of body 102 and bolt 103 is then assembled to port 101. Sealing of the body/bolt assembly is accomplished with copper (typically) washers 104 and 105, which are placed each side of body 102. Large torques (applied in the radial direction of arrow T of FIG. 4) are employed to obtain a seal between components, which sometimes cannot be sealed. A common failure of this assembly is to snap the bolt or strip the threads of the port while striving to attain sealing contact.
The "banjo" fittings are useful because of their inherent assembly benefits in situations where time taken to assemble and ease of access to components are important considerations. "Banjo" fittings are assembled from the front, and assembly can be effected with power tools. This is not true of other fittings which perform the function served by "banjo" fittings, that of supplying fluid to a component through a 90 degree change of direction or other similar reorientation of fluid flow.
A drawing of an assembled prior art "banjo" fitting is shown in FIG. 4, where body 102, shown in partial cut-away, is recessed internally to create flow chamber 124 for pressurized fluid. A recessing operation is required to be done to body 102, rather than in bolt 103, in order to retain as much tensile strength in bolt 103 as possible. If bolt 103 were reduced in diameter to create a flow chamber, insufficient material would remain in bolt 103 to withstand the massive assembly torques required to obtain a seal. The recessing operation required for body 102 is expensive and difficult to control. Also, bolt 103 must be made of relatively high tensile material in order to resist tensile failure due to high torques, and such materials are hard to machine which further complicates the manufacture of prior art "banjo" fittings.
One problem with prior art "banjo" fittings involves the four potential leak-paths in any standard banjo fitting, one on each side of the metal washers 104 and 105 which are located at sealing points 120, 121, 122 and 123 of FIG. 4. Also a problem is that massive torques are required to attain a seal, which in turn, requires a high tensile strength bolt, and an internally recessed body. These are expensive requirements for the manufacture of the "banjo" fitting.
An additional problem involves the lack of any secondary seal in the port interface. Should one of the four metal-to-metal interfaces develop a leak, the only way of overcoming it is to impart greater torque to the assembly. This regularly leads to tensile failures of bolts or stripped threads on bolts or in ports.
Further problems involve shape, size and alignment of the prior art "banjo" fittings. Flow characteristics within the fitting are primarily derived from consideration of tensile strengths rather than from system demand. This often leads to flow restrictions which are not desirable. Also, the face-to-face association of components requires close control in order to form an adequate seal. Concentricity and squareness of through-bores on bodies must be carefully maintained in production, and aligned accurately during assembly, if a seal is to be obtained.
What is needed is an improved hydraulic coupling which utilizes the elastic properties of the materials.
Also needed is an improved hydraulic coupling which does not excessively deform o-rings in the coupling.
A further need is for an improved hydraulic coupling which includes secondary seals.
An additional need exists for an improved hydraulic coupling for refrigerant systems which includes metal-to-metal seals.
A still further need exists for an improved hydraulic coupling which minimizes the number of potential leak paths.
Yet another need exists for an improved hydraulic coupling which requires less torque for assembly.